net: txgbe: support switching mode to 1000BASE-X and SGMII

Disable data path before PCS VR reset while switching PCS mode, to prevent
the blocking of data path. Enable AN interrupt for CL37 auto-negotiation.

Signed-off-by: Jiawen Wu <jiawenwu@trustnetic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>